1. Field of the Invention
The invention relates to a dark metallic effect pigment which has diffractive structures, to a process for preparing these pigments, and to their use.
2. Description of Related Art
Metallic effect pigments have been used for many years in coatings in order to generate a metallic effect.
Conventional metallic effect pigments consist of platelet-shaped metallic pigments whose effect derives from the directed reflection of incident light at the metallic pigments of lamellar form which are oriented in parallel in the respective application medium.
Typical fields of application of the metallic effect pigments are the coatings industry, especially the automotive industry, the printing industry, and the plastics industry.
Metallic effect pigments produced by PVD techniques have been known for some considerable time. They are notable for extremely high luster, immense covering power, and unique optical properties. Owing to their low thickness of around 30 to 70 nm and their extremely smooth surfaces, they have a tendency, following application, to conform very closely to the substrate. If the substrate is very smooth, the result is virtually a mirrorlike appearance.
Likewise known are metallic effect pigments, produced by PVD techniques, that have a regular embossed structure. This structure is able to act as an optical lattice, and breaks incident light down into its spectral colors. As a consequence of this, these pigments generate a colorfully shimmering color effect (“rainbow effect”) in unison with the typical properties of metallic pigments such as high luster, high brightness, and very good hiding power. Pigments of this kind are disclosed for example in U.S. Pat. No. 5,624,076 and also in U.S. Pat. No. 6,692,830 B2.
Cosmetic applications of such pigments are disclosed in EP 1 694 288 B1. They are in commerce under the trade names Holographic® or Metalure Prismatic®.
The company Flex (Santa Rosa, Calif., USA) offers pigments under the trade name Spectraflair®. The structure and production of these pigments are described in Barbara Parker “Advances in Interference Color”, in the conference volume of the Color Cosmetic Summit, Montreal (2003). These pigments are multilayer metallic effect pigments generated by PVD techniques, having a central core of aluminum with layers of MgF2 applied to it. Embossed onto these layers of low refractive index is a lattice with a defined spacing of the lattice structures. These pigments generate extremely strong rainbow effects.
In order to generate a dark metallic effect and at the same time a rainbow effect, the known metallic effect pigments with rainbow effect can be tinted with commercially available black pigments. A disadvantage, however, is that such formulations always have a disadvantageous brown tinge. This occurs particularly at shallow angles of observation and/or incidence.
EP 1 522 606 A1 describes the production of a film comprising black aluminum oxide. Neither metallic effect pigments nor multilayer structures are disclosed therein. The films that are disclosed therein have no significant metallic effect with luster and flop.
U.S. Pat. No. 4,430,366 describes the production of films which comprise a sequence of inherently homogeneous layers of metal and metal oxide. Here, again, there is no mention of metallic effect pigments.
DE 69601432 T2 relates to a method for thermal generation of an image on a substrate, where an oxygen-containing, black aluminum layer is applied in such a way that the layer has an optical transmission of at least 0.3 at a wavelength between 200 and 1100 nm. This document, again, does not relate to the provision of metallic effect pigments.
EP 1 144 711 B1 discloses a method for producing reflective color pigments, in which, to a reflection layer, at least one layer which brings about a color change and comprises a transparent material having a refractive index of greater than 1.8, typically metal oxide, and a light-absorbing metal is applied with simultaneous evaporation, the light-absorbing metal being different to the metal of the metal oxide. In terms of process engineering, the method is very hard to control.
DE 10 2007 007 908 A1 discloses dark metallic effect pigments which are produced by PVD techniques. They have a largely homogeneous composition and possess a relatively high oxygen content of 25 to 58 atom %. The layer is dark because the metal is in the form of small metal nanoparticles in dispersion in metal oxide. Metal effect pigments of this kind produce dark and yet highly lustrous metallic effect pigments with a pronounced light/dark flop. The pigments, however, do not exhibit any rainbow effect.
WO 2009/012995 A1 discloses three-layer metallic effect pigments which are likewise able to produce very dark metallic effect pigments. Here again, however, there is no rainbow effect.
It is an object of the present invention to provide a dark and/or colored metallic effect pigment featuring metallic luster and a rainbow effect. In applications pigmented with these pigments, they are not to exhibit any brown tinge, particularly at shallow angles of incidence and/or observation. With particular advantage, instead, a blue tinge is to be visible.